This invention relates to apparatus and an associated process for manufacturing a semiconductor wafer. More specifically, this invention relates to apparatus with a shield for the back of the wafer to prevent contamination by the photoresist and it relates to a process that is used with the shield.
Processes and apparatus for carrying out manufacturing steps on semiconductor wafers are well known, but it will be helpful to review the features and terminology that particularly apply to this invention. The wafer is held in a chuck while semiconductor devices are formed in an array on the exposed surface of the wafer. In one of the steps, the exposed surface of the wafer is coated with a photosensitive resist. The other surface is called the wafer back or more simply the back.
There is a known problem in this art that the wafer back can become contaminated with the resist. This contamination is more troublesome as devices on the wafer are made smaller.
The coating step may be repeated at several steps of the overall process, and back contamination can be carried into the coating station. Wafer back contamination can also be attributed to the structure of the coating apparatus and to the process for using the apparatus.
U.S. Pat. No. 4,982,694 shows a wafer 11, a wafer chuck 10 and a nozzle 50 for spraying a solvent on the back side of the wafer. This patent also discusses the speed of the spinner.
U.S. Pat. No. 5,116,250 spins the wafer in the range of 1000 to 6000 rpm, uses the air flow through the apparatus to help remove the vapor, and has a form of mask for the wafer back.
U.S. Pat. Nos. 5,075,256, 4,393,807 and 4,510,176 also discuss the problem.
One feature of this invention is an improved shield that protects the wafer back from the resist vapor. The shield is located where vortex like air currents could otherwise deposit the resist vapor on the wafer back.
The shield has the general shape of a cylinder that is open at the top. The bottom provides an attachment to a conventional part of the wafer coater and also forms part of the shield. The sides are arranged to extend close to the wafer back at a radius just less than the radius of the wafer. A suitable separation is no greater than 2.0 mm and the overhang of the wafer beyond the cylindrical sides of the shield is 4 to 6 mm.
In one feature of the improved process, the spindle of the wafer chuck is not rotated at more than 1200 revolutions per minute in any of the wafer spinning operations. As is known, limiting the speed reduces the amount of resist vapor produced while the wafer is being spun.
In another feature of the improved process, the conventional step of washing the wafer back with a solvent is performed only at the end of the other steps.
Other features and advantages of the invention will be understood from the description of the preferred embodiment.